Regulation of Tristetraprolin Expression by Interleukin-1β and Dexamethasone in Human Pulmonary Epithelial Cells: Roles for Nuclear Factor-κB and p38 Mitogen-Activated Protein Kinase

The mRNA-destabilizing protein tristetraprolin (TTP) negatively regulates adenine- and uridine-rich element (ARE)-containing mRNAs. In A549 pulmonary cells, TTP mRNA and both a similar to 40- and a similar to 45-kDa phosphorylated version of TTP protein were rapidly induced in response to interleukin (IL)-1 beta. Analysis with I kappa B alpha Delta N, a dominant version of inhibitor of kappa B alpha (I kappa B alpha), as well as dominant-negative and small-molecule I kappa B kinase (IKK) inhibitors demonstrated that IL-1 beta-induced TTP is nuclear factor-kappa B (NF-kappa B)-dependent. Likewise, TTP expression and formation of the similar to 45-kDa phosphorylated form of TTP are blocked by the p38 mitogen-activated protein kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580). By contrast, and despite a 3- to 4-fold induction of TTP mRNA, the anti-inflammatory glucocorticoid dexamethasone only modestly induced expression of the similar to 40-kDa form of TTP. In the context of IL-1 beta, dexameth-asone exerted a marginal repressive effect on TTP mRNA expression and more considerably reduced TTP protein. Given a requirement for p38 MAPK in the induction of TTP by IL-1 beta, this repressive effect may be explained by repression of the p38 MAPK pathway by dexamethasone. Knockdown of TTP protein by siRNA elevated IL-1 beta-induced expression of granulocyte macrophage-colony-stimulating factor (GM-CSF) and IL-8, demonstrating a role for TTP in feedback control. Likewise, knockdown of TTP increased GM-CSF expression in the presence of IL-1 beta plus dexamethasone, suggesting that feedback control by TTP also occurs in the context of IL-1 beta plus dexamethasone. Taken together, our data demonstrate that NF-kappa B and p38 MAPK are critical to the induction of TTP by IL-1 beta and that TTP induction provides feedback control of the ARE-containing genes GM-CSF and IL-8.